Sound transducers like, for example, headphones or loudspeakers are widely used for presenting audio signals to listeners. In some cases, the sound transducers are sold together with the equipment providing the audio signals to be presented by said sound transducers. However, in many cases, the sound transducers are bought separately by the customers, which often results in a degradation of the audio quality.
In the following, some problems will be outlined taking reference to headphones, which are one possible example of a sound transducer.
First of all, some general characteristics of headphones will be described. There are different types of headphones used in consumer and professional audio: insert ear-phones (intra-canal), ear-buds (intra-concha), on-ear (supra-aural) and over-the-ear (circum-aural). In mobile communications, headphones are often combined with microphones in one device in order to do hands-free voice calls. For simplicity, these “headsets” will be also referred to as headphones in this document.
Headphones are produced using various technologies and materials of different quality levels. These differences lead to different sound characteristics.
This is mainly due to the varying frequency responses produced by different headphones (see, for example, FIG. 9, which shows a frequency response of different headphones). Moreover, reference is also made to document [1]. For example, in the graphic representation 900 according to FIG. 9, an abscissa 910 describes a frequency (in the unit of Hertz) in a logarithmic manner. An ordinate 920 describes a level (or relative level) in the unit of decibels in a logarithmic manner. As can be seen, a curve 930 describes a so-called “diffuse-field” frequency response according to international standard ISO-11904-1. A second curve 932 describes the frequency response of a “high quality” headphone. A third curve 934 describes a frequency response of a “low-cost” headphone. As can be seen, the “high-quality” headphone comprises a frequency response which approximates the “diffuse-field” frequency response better than the frequency response of the “low-quality” headphone.
Moreover, it should be noted that the frequency response of a headphone is an important component of its perceived quality (see, for example, reference [2]).
Ideally, the headphones should be capable of providing a frequency response that follows a defined target curve, for example, diffuse field equalization (see, for example, reference [3]). Headphones that have a frequency response which strongly differs from an ideal frequency response are typically judged to have a bad audio quality.
The frequency response of a headphone can be identified, for example, by a measuring on a defined coupler (see, for example, reference [4]). The frequency response describes how much sound pressure is produced in the ear canal when a specific level electric voltage is fed to the headphones. The level of sound pressure is frequency-dependent.
Measuring these frequency responses of headphones is quite challenging. A dummy head equipped with ear-simulators or an acoustic coupler, special audio measurement hardware and software and appropriate knowledge is highly recommendable or even mandatory for proper results. Hence, measuring frequency responses of headphones should advantageously be made by professionals and not by consumers and/or end-users.
In the following, some conventional filters for headphones will be described. However, it should be noted that the filters can be used for any type of sound transducer.
The audio quality of headphones can be significantly improved. Therefore, the signal that is later fed to the headphones may be preprocessed. Each headphone shows a unique frequency response (see, for example, FIG. 10a). A specific filter for this headphone (see, for example, FIG. 10b) compensates for the imperfect frequency response, as described for example in reference [5]. This process is referred to as headphone equalization. Hence, the ideal quality of these headphones is raised by adapting the frequency response to a certain design goal (see, for example, FIG. 10c).
In the following, some details will be explained taking reference to FIG. 10, which shows a scheme for the generation of discrete filters for specific headphones. FIG. 10a shows a frequency response of a headphone. An abscissa 1010 describes a frequency in Hertz, and an ordinate 1012 describes a magnitude of the frequency response, for example, in a logarithmic form in decibels. A curve 1014 describes the frequency response of an example headphone. FIG. 10b shows a filter for the frequency response according to FIG. 10a to achieve a target curve according to FIG. 10c. In other words, FIG. 10b shows a frequency response of a filter or equalization filter, which can be used to achieve the overall target frequency response according to FIG. 10c when used to equalize an audio signal provided to the headphone having the frequency response according to FIG. 10a. An abscissa 1020 describes a frequency (for example, in Hertz) and an ordinate 1022 describes a (relative) magnitude of the filter response (for example, in the unit of decibels). A curve 1024 describes the frequency response of the equalization filter. FIG. 10c describes a target frequency response curve. An abscissa 1030 describes a frequency in Hertz, and an ordinate 1032 describes a magnitude of the target frequency response, for example, in decibels. A curve 1034 describes the target frequency response, which may, for example, approximate the diffuse-field frequency response according to ISO-11904-1.
It should be noted that the frequency response of an equalization filter, which filters (or equalizes) an audio signal to be output via a specific headphone, may be determined as the “difference” (or, more precisely, the quotient) between the target frequency response (as described, for example, in FIG. 10c) and the actual (measured) frequency response of the headphone currently under consideration (as shown, for example, in FIG. 10a). In other words, the target frequency response of the filter (equalizer filter) can be determined on the basis of the knowledge of the target frequency response curve and the actual frequency response curve of the headphone under consideration. Since the actual frequency response curves of different headphones differ, the associated equalization filter frequency response curves also differ.
Moreover, it should be noted that the technique described in reference [5] can be used to create different discrete filters for various headphones. Nevertheless, the conventional concepts for headphone equalization typically demand high skills from the operator and are hardly usable by inexperienced end users.
WO2010/138309 [8] describes an audio signal dynamic equalization processing control which, however, is computationally very complex and does not allow for the definition of temporally constant signal-independent equalization.
To summarize, it is conventionally not possible or very difficult for an end user to properly adjust the filter coefficients of an equalization filter to obtain a good hearing impression using headphones.
Accordingly, there is a desire to create a concept which makes it easier for an end user to obtain a reasonably good set of filter coefficients for an equalization filter to improve the (effective) frequency response of a given sound transducer (like, for example, a headphone) using an equalizer.